Pneumothorax refers to air accumulation in the chest cavity. As odd as this sounds, the chest cavity must have a vacuum (no air) to work properly. There should be no air pressure outside the lungs. This enables them to inflate when the muscle of the chest are relaxed and it expands. The air is then forced out of the chest by contraction of the muscle of the chest. When the lung is damaged and leaks air, it collapses the lung. The same thing happens when there is a hole in the chest allowing air in. Either situation can be the cause of pneumothorax after a traumatic event like being hit by a car. If there is only a small amount of air accumulated it can be reabsorbed. This is commonly the case with blunt trauma in which the lung ruptures, then heals itself quickly. If there is a lot of air or continuous leakage of air, it is usually necessary to place a drain in the chest to remove the air and maintain a vacuum. There are several ways of doing this to ensure continuous evacuation of air. We use a Heimlich valve (same guy as the Heimlich maneuver, I'm sure) attached to a silicon tube in the chest.